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<title>HDF5 Library Release Version Numbers</title>
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<h2>Contents</h2>

<ul>
  <li><a href="#Introduction">Introduction</a></li>
  <li><a href="#Definitions">Definitions</a></li>
  <li><a href="#TrunkReleaseBranchesFeatureBranches">The Trunk, Release Branches, and Feature Branches</a></li>
  <li><a href="#VersionSupportFromTheLibrary">Version Support from the Library</a></li>
  <li><a href="#UseCases">Use Cases</a></li>
<ul>

<li><a href="#ApplicationVersionChecking">Application Version Checking</a></li>
<li><a href="#ConditionalInclusions">Conditional Inclusions or Exclusions Based on the Version</a></li>
<li><a href="#SpecifyingAFormat">Specifying a Format</a></li>

</ul>
  <li><a href="#SharedLibraryVersionNumbers">Shared Library Version Numbers</a></li>
</ul>

<h2><a name="Introduction">Introduction</a></h2>

<p>HDF5 software is updated on a regular basis. These updates, known
as releases, range in scope and size from small to large. Some updates
may only fix bugs, and some updates may require a change in the format
of the data file. The version numbers that are applied to updates give
information about the kinds of changes made in the updates. This Tech
Note describes what the version numbers mean.</p>

<p>Note that this document describes release version numbers for the
HDF5 Library. For more information, see the
<a href="#SharedLibraryVersionNumbers">Shared Library Version Numbers</a> section at the end of this document.</p>

<h2><a name="Definitions">Definitions</a></h2>

<p>Each software release of the HDF5 Library is labeled with a version
  number. The version number is a set of three integers written as HDF5-1.2.3,
  HDF5 version 1.2 release 3, or HDF5 Release 1.2.3. The version number
  might also include text. A patch version might be labeled HDF5-1.2.3-patch1.
  The '5' in "HDF5" is part of the product name and will not change during
  the life of the project.</p>
 
  <p>The key components in HDF5 Library version numbers are the major version
  number, the minor version number, the release number, and an optional text
  string. </p>
 
  <p>The first integer in a version number is the <b>major version
  number</b>. This integer increments when there is an extensive change
  to the file format or library API. Such a change may require files to
  be translated and will likely require applications to be modified.  </p>

  <p>The second integer, 2 in the examples above, is the <b>minor version
  number</b>. This number is incremented when there are new features that
  require a change in the file format. For example, a change in file format
  was required during the change from version 1.6 to version 1.8. Stable
  released versions of the library are given <i>even</i> minor version
  numbers such as 1.6 and 1.8 while <i>odd</i> minor version numbers such
  as 1.7 and 1.9 are used on the trunk for major development. See the
  section below for more information.</p>

  <p>The third integer, 3 in the examples above, is the <b>release
  number</b>. A change in this number indicates that the library has
  been updated. The updates might include bug fixes, performance
  improvements, and new features that do not require a file format
  change. </p>
 
  <p>A version number might also include some text. The two current
  possibilities are <b>patch</b> and <b>snap</b>. A patch version might
  be made to a released version to make available a feature or a bug
  fix. In the figure below, a patch to the 1.8.5 release is labeled
  1.8.5-patch1. A snapshot is an intermediate posting of the software
  in a branch or in the trunk. Snapshots are made available so that users
  may begin to test changes in the software that affect their software.
  The changes may range from bug fixes to new features. Snapshots are made
  and released regularly. How regularly depends on whether the software
  passes the tests done on each build. A possible version number for a
  snapshot might be 1.9-snap81. This version would hold the 81st snapshot
  off the 1.9 development branch (the current trunk). For the
  snapshots are available at 
  <a href="https://github.com/HDFGroup/hdf5/releases/tag/snapshot">https://github.com/HDFGroup/hdf5/releases/tag/snapshot</a>.</p>

<h2><a name="TrunkReleaseBranchesFeatureBranches">The Trunk, Release Branches, and Feature Branches</a></h2>

  <p>The HDF Group uses a version control system to manage the HDF5
  project. Within the system, a trunk and branches are used to track
  changes. The version numbers described above identify where a given
  piece of software was produced in the system. The figure below shows
  the general scheme.</p>

<table align="center" width="500" border="0">
    <tbody><tr valign="center" align="center">
        <td>
        <hr color="green" size="3">
        <img height="400" src="trunk_branches.jpg" alt="The trunk, release branches, and feature branches"></td>
        </tr>
    <tr valign="center" align="left">
        <td>
        <hr color="green" size="1">
        <b>Figure 1.</b> The trunk, release branches, and feature branches
        </td>
        </tr>
</tbody></table>
   
  <p>The <b>trunk</b> is the center of the system. New features are
  implemented in <b>feature branches</b> and aggregated in the trunk.
  <b>Release branches</b> are then created from the trunk.</p>
 
  <p>The minor version number of the trunk is always an odd number. From
  the time of Release 1.8.0 to the first 1.10 release, the trunk will be
  version 1.9. The trunk was version 1.7 from the time of release 1.6.0
  until the first 1.8 release.</p>
   
  <p>Projects that add new features, bug fixes, and performance improvements
  are developed on <b>feature branches</b>. When a project is completed,
  its feature branch is merged into the trunk. In the figure above, the
  merging of a feature branch is represented by a dashed arrow from the
  feature branch to the trunk. If a feature requires a file format change,
  then the feature will stay in the trunk until the next significant
  release. This would mean in the figure above that the new feature would
  be released in a future 1.10 release branch. If a feature does not
  require a file format change, then it might be merged into one or more
  release branches. This would mean in the figure above that the new
  feature could be merged into the 1.8 branch and could be included in
  the 1.8.6 release. If the feature was added to the 1.8.5 branch, then a
  patch version might be released.</p>
   
  <p><b>Release branches</b> hold software that is distributed to general
  users. In the figure above, a few release branches are shown below the
  trunk. Work is done in release branches for a period of time. Branches
  further from the trunk have less work done in them. For example, a patch
  branch such as 1.8.5-patch1 may contain only one or two changes. A release
  branch such as 1.8.5 may contain a number of bug fixes and new functions,
  but these changes are small in number compared to the number of changes in
  the 1.8 branch. </p>
   
  <p>We aim to make available to the public two maintenance releases a year.
  The releases occur usually in the spring near May 15 and in the fall near
  November 15. If two release branches are being maintained, then
  maintenance releases may be made for each release branch. For example,
  there was a time when both the 1.6 and 1.8 branches were actively
  maintained. In one maintenance release, the 1.6.10 and 1.8.4 versions were
  released at the same time. The 1.6 and 1.8 branches were both actively
  maintained to give early adopters access to new features and to give most
  users plenty of time to make the change to 1.8 software from 1.6.</p>
   
  <p>As we improve any branch, we consider the effect of any change on the
  readability of objects. Applications built, for example, with version
  1.8.5 will be able to read data files written with any prior version
  of the library. So, a 1.8.5 application will be able to read a dataset
  written with 1.4.5. A 1.8.5 application may be able to read a dataset
  written under the 1.8.7 library if no new features, features not known
  to 1.8.5, were used.

<h2><a name="VersionSupportFromTheLibrary">Version Support from the Library</a></h2>

  <p>The library provides macros and functions to query and check
  version numbers.</p>

  <p>The following constants are defined in the file <code>H5public.h</code>
  and determine the version of the include files.</p>

  <ul>
    <li><code>H5_VERS_MAJOR</code></li>
    <li><code>H5_VERS_MINOR</code></li>
    <li><code>H5_VERS_RELEASE</code></li>
    <li><code>H5_VERS_SUBRELEASE</code></li>
    <li><code>H5_VERS_INFO</code></li>
  </ul>
 
  <p>The table below describes some of the function calls and macros
  that can be used to query and check version numbers.</p>

<table width="600" cellspacing="0" align="center">
    <tbody><tr valign="bottom">
        <td colspan="2" align="left" valign="bottom">
        <b>Table 1. Version function calls and macros</b></td>
        </tr>
    <tr><td colspan="2"><hr color="green" size="2"></td></tr>
    <tr valign="top">
        <td width="35%">
        <b>Function Call or Macro</b></td>
        <td width="65%">
        <b>Comments</b></td>
    </tr><tr><td colspan="2"><hr color="green" size="1"></td></tr>
    <tr valign="top">
        <td><code>H5get_libversion</code></td>
        <td>This function returns through its arguments the version
        numbers for the library to which the application is linked.</td>
        </tr>
    <tr><td colspan="2"><hr color="green" size="1"></td></tr>
    <tr valign="top">
        <td><code>H5check</code></td>
        <td>This macro uses the <code>H5check_version</code> function
        to verify that the version number of the HDF5 include file used
        to compile the application matches the version number of the
        library to which the application is linked. This check occurs
        automatically when the first HDF5 file is created or opened and
        is important because a mismatch between the include files and the
        library is likely to result in corrupted data and/or segmentation
        faults. If a mismatch is detected, the library issues an error
        message on the standard error stream and aborts. </td>
        </tr>
    <tr><td colspan="2"><hr color="green" size="1"></td></tr>
    <tr valign="top">
        <td><code>H5check_version</code></td>
        <td><p>This function is called by the <code>H5check</code> macro
        with the include file version constants. The function compares
        its arguments to the result returned by <code>H5get_libversion</code>.
        If a mismatch is detected, it prints an error message on the standard
        error stream and aborts.</p>
       
        <p>The behavior of this function can be modified by the
        <code>HDF5_DISABLE_VERSION_CHECK</code> environment variable. Setting
        the environment variable to a value of =911=92 will issue a
        warning but continue without aborting. Setting the environment
        variable to a value of =912=92 will suppress the warning
        and continue silently without aborting.</p></td>
        </tr>
    <tr><td colspan="2"><hr color="green" size="1"></td></tr>
    <tr valign="top">
        <td><code>H5_VERSION_GE</code> and
        <code>H5_VERSION_LE</code></td>
        <td>These macros compare the version of the HDF5 library being used
        against the version number specified in the parameters. At compile
        time, they can be used to conditionally include or exclude code
        based on the library's version.</td>
        </tr>
    <tr><td colspan="2"><hr color="green" size="1"></td></tr>
    <tr valign="top">
        <td><code>H5Pset_libver_bounds</code></td>
        <td>This function can be used to control the versions of the object
        formats that will be used when creating objects in a file.  </td>
        </tr>
    <tr><td colspan="2"><hr color="green" size="2"></td></tr>
</tbody></table>
 
  <p>For more information on these and other function calls and macros,
  see the <a href="https://\DOXURL/_r_m.html"> <cite>HDF5 Reference Manual</cite></a>. </p>

<h2><a name="UseCases">Use Cases</a></h2>

  <p>The purpose of this section is to describe how some of the version
  functions, macros, and constants might be used. </p>

<h3><a name="ApplicationVersionChecking">Application Version Checking</a></h3>

  <p>Suppose first that a developer builds an application that will read
  from and write to an HDF5 file. When the application is compiled, a
  version of the HDF5 Library such as 1.8.6 will be used. The version
  constants (<code>H5_VERS_MAJOR</code>, <code>H5_VERS_MINOR</code>,
  and <code>H5_VERS_RELEASE</code>) are included in the application when
  it is compiled. </p>

  <p>Suppose next that a user gets a copy of the application and starts it
  up on a workstation. The executable is put into memory along with the
  HDF5 Library. However, an application may only work successfully with
  the version of the library with which the application was compiled. In
  other words, the version of the library that is loaded when the applicati=
on
  is started must be the same version as the version of the library with
  which the application was compiled. This is verified by the library when
  the first HDF5 API routine is called. If an application wants to confirm
  early in its startup procedure that the version of the library that will
  be loaded into memory at the workstation will work with the application,
  then it can use the <code>H5get_libversion</code> and
  <code>H5check_version</code> function calls. </p>

<h3><a name="ConditionalInclusions">Conditional Inclusions or Exclusions Based on the Version</a></h3>

  <p>The <code>H5_VERSION_GE</code> and <code>H5_VERSION_LE</code> version
  macros compare the version of the HDF5 Library being used against the
  version number specified in the parameters. At compile time, they can be
  used to conditionally include or exclude code based on the library's
  version. For example, the link functions, <code>H5Lxxx</code>, are
  new in version 1.8, and some group functions, <code>H5Gxxx</code>,
  are deprecated in 1.8. With the <code>H5_VERSION_GE</code> macro, an
  application could use <code>H5Ldelete</code> if the library version is
  1.8.0 or greater, or it could use <code>H5Gunlink</code> if the library
  version is less than 1.8.0. </p>

<h3><a name="SpecifyingAFormat">Specifying a Format</a></h3>
  <p>Suppose a data file has three datasets. It is possible that the three
  datasets were added to the data file with applications using different
  versions of HDF5. The different versions could be 1.4.5, 1.6.10, and
  1.8.6. If another dataset is written to the data file, then it will be
  written by default in the oldest format possible that has all of the
  features needed to successfully write the dataset. If a newer feature
  such as compact storage, a new parameter for a function, or a partially
  compressed dataset is used, then a newer format will be used.
  <code>H5Pset_libver_bounds</code> could be used to specify the oldest
  format used. In the situation above, the owners of the data file might
  want all data written to the file in the future to be in a 1.8 format
  rather than 1.6 or 1.4. </p>

<h2><a name="SharedLibraryVersionNumbers">Shared Library Version Numbers</a></h2>

<p>HDF5 shared libraries utilize the
<a href="https://www.gnu.org/software/libtool/manual/libtool.html#Versioning">
libtool versioning system</a> in order to indicate interface
compatibility between maintenance releases of HDF5. While we always
attempt to maintain interface compatibility between minor maintenance
release versions of HDF5, if we are forced to break interface
compatibility in order to resolve a critical defect within the
library, then the library interface version attached to the shared
libraries for a given release will be incremented accordingly.</p>

<p>Please note that this libtool version number for interface
compatibility is unrelated to the HDF5 release version for a given
release.</p>

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